Simon Singh breaks down cryptic messages for the teenage set in The Code Book: How to Make It, Break It, Hack It, Crack It, an adaptation of his bestselling adult title The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptology. He covers actual instances of codebreaking, from its role in the plan to execute Mary, Queen of Scots, to the Navajo code talkers of WWII. (Mar.)